FIGS. 1A and 1B respectively show a plan view and a perspective view of an exemplary electronic device 100. As shown in FIGS. 1A and 1B, the electronic device 100 comprises a plurality of electrical lead pads 102. These lead pads 102 are used for the connection of the electronic device 100 to other electronic components.
To check if an electronic device operates as desired, the electronic device is usually subjected to various tests after its fabrication. This is typically performed by probing lead pads of the electronic device. To elaborate, probe pins (or in other words, contact pins) are brought into contact with the lead pads of the electronic device to connect the device to a tester. The tester sends signals to the electronic device and analyzes the electronic device's responses to determine if the device is working properly. As the signals to the device and the responses from the device are relayed via the connections between the lead pads and the probe pins, it is important to ensure proper electrical contact between the lead pads and the probe pins.
Ideally, the probe pins should contact the centres of the respective lead pads. FIG. 2 shows such an ideal situation. In particular, FIG. 2 shows a plan view of an electronic device 200 having a plurality of probe marks 204 at the centres of respective lead pads 202. The probe marks 204 indicate positions of the probe pins when the probe pins are brought into contact with the lead pads 202. Indentations with a particular depth are formed by the probe pins, depending on the force at which the device 200 is urged against the probe pins, the material composition of the lead pads and/or the number of times such urging occurs.
In order to get as close to the ideal situation as possible, the positions of the probe pins may be adjusted during the testing process. Conventionally, this is done by determining the positions of the probe marks on the lead pads after testing each electronic device and based on this determination, adjusting the positions of the probe pins for the next electronic device. To elaborate, after testing an electronic device that is held on a turret of a test handler, a camera captures an image of the electronic device to try to detect the positions of the probe marks on the lead pads of the device. The difference between the positions of the probe marks and the centres of respective lead pads is then calculated and used to adjust the positions of the probe pins. The turret of the test handler then retrieves a next electronic device for testing using the adjusted positions of the probe pins. The afore-mentioned steps are repeated for each electronic device.
A problem with the above approach is that it is often difficult to detect accurately the positions of the probe marks from the image captured after testing the electronic device. This is due to “noise” in the captured image which may be caused by unrelated features and/or defects in the electronic device. In turn, a number of tests with the probe pins in less than ideal positions may have to be performed before the difference in the positions of the probe marks and the centres of respective lead pads can be accurately calculated. This therefore results in low yield as many electronic devices tested at the beginning of the testing process may be wrongly classified as defective.
To overcome the above problem, methods to reduce the noise in the captured images have been developed. One such method involves performing various operations on the captured image to obtain one or more inspection result images, each based on a unique image characteristic or a unique combination of image characteristics. Each inspection result image is then correlated with a reference image to determine which image characteristic or combination of image characteristics is likely to provide the necessary contrast. The image characteristic or combination of image characteristics most likely to provide the necessary contrast is then used for the processing and inspection of subsequent captured images. Although such a method may help increase the accuracy in determining the probe marks' positions, it is unlikely that it can sufficiently improve the production yield. This is because there are many potential sources of noise in each captured image and the captured images of a plurality of electronic devices can differ from one another by a non-negligible amount. Therefore, it is unlikely that the method described above can sufficiently compensate for the noise in all the captured images to the extent that the probe marks positions can be detected at a high enough accuracy for every image.